Method for inhibiting tin whisker growth

ABSTRACT

A uniform copper-tin compound layer is electrochemically deposited on a surface of a copper-based base structure. A tin-based film is then formed on the copper-tin compound layer. The uniform copper-tin compound layer provides a barrier that effectively inhibits tin whisker growth.

TECHNICAL FIELD

Embodiments relate to tin-based plating applied to metallic devices and components (such as, for example, electrical connection terminals or lead frames for integrated circuit devices) and, in particular, to a method for plating such metallic devices and components so as to inhibit tin whisker growth.

BACKGROUND

Reference is made to FIGS. 1A-1C which show steps of a prior art process for plating metallic devices and components for inhibiting growth of whiskers. See, for example, U.S. Pat. No. 7,931,760 and United States Patent Application Publication No. 2008/0316715, both of which are incorporated herein by reference. In FIG. 1A, a metallic device or component base 10 is made of copper or a copper alloy. The metallic device or component base 10 may, for example, comprise an electrical connection terminal or a lead frames for an integrated circuit device. A tin or tin-based film 12 is formed over the base 10 (for example, using an electroless deposition process). The film 12 has a preferred thickness. A thermal treatment is then applied at a desired temperature and for a desired length of time as a function of the preferred thickness in order to diffuse copper from the base 10 into the film 12. As an example, the desired temperature may be in the range of 90-160° C., and the desired length of time may be in the range of 30 to 90 minutes. Due to the film having the preferred thickness, the applied thermal treatment converts substantially all (preferably, completely all) of the tin or tin-based film 12 into a stable copper-tin compound (Cu—Sn alloy) barrier film 14 as shown in FIG. 1B. As an example, the barrier film 14 may comprise Cu₃Sn. Next, a tin or tin-based film 16 is formed over the barrier film 14 as shown in FIG. 1C. The stability of the Cu₃Sn barrier film 14 prevents formation of uncontrolled intermetallic Cu₆Sn₅ growth, even in the presence of the overlying tin or tin-based film 16. As a result, tin whisker growth is inhibited.

In an alternative implementation, the thermal treatment is instead performed after formation of the tin film 16 (referred to by those skilled in the art as a “post bake”).

SUMMARY

In an embodiment, a method comprises: electrochemically depositing a uniform copper-tin compound layer on a surface of a copper-based base structure; and forming a tin-based film on the copper-tin compound layer.

In an embodiment, an apparatus comprises: a copper-based base structure; a uniform electrochemically deposited copper-tin compound layer on a surface of the copper-based base structure; and a tin-based film on the copper-tin compound layer.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the embodiments, reference will now be made by way of example only to the accompanying figures in which:

FIGS. 1A-1C show steps of a prior art process for plating metallic devices and components for inhibiting growth of whiskers; and

FIGS. 2A-2C show steps of a process for plating metallic devices and components for inhibiting growth of whiskers.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is made to FIGS. 2A-2C which show steps of a process for plating metallic devices and components for inhibiting growth of whiskers. In FIG. 2A, a metallic device or component base 20 is made of copper or a copper alloy. The metallic device or component base 20 may, for example, comprise an electrical connection terminal or a lead frames for an integrated circuit device. Using an electroplating process, a uniform layer 22 of a copper-tin compound (Cu—Sn alloy) is deposited on the base 20 as shown in FIG. 2B. In this context, the term “uniform” as relates to the layer 22 means that the layer has a same thickness, same morphology and same stoichiometry across the entire surface of the base 20. This is contrasted to intermetallic growth of Cu—Sn alloy where that growth is non-uniform due to localized growth at grain boundaries. The use of the electroplating process is critical to ensuring uniform growth of the Cu—Sn alloy as distinguished from natural growth of the Cu—Sn alloy which tends to segregate at grain boundaries.

In terms of thickness, the layer 22 may, for example, have a thickness on the order of 1 μm (more generally, a thickness in a range from 0.5 μm to 1.0 μm). In this regard, it is noted that a thickness less than 0.5 μm may not be sufficient to inhibit whisker growth. Conversely, a thickness exceeding 1.0 μm can lead to formation of a fragile interface with increased risk of cracking.

In terms of morphology, the layer 22 exhibits a homogenous granular structure with a smooth surface. The layer 22 may be characterized as having a relatively fine grain structure (for example, less than or equal to about 1 μm) with a nodular formation and minimal to no presence of dendritic formations on preferential locations.

In terms of stoichiometry, the layer 22 may comprise Cu₆Sn₅, for example with a weight percent of copper at 56% and a weight percent of tin at 44%. The proportion of metallic elements in the layer 22 is consistent across the lateral extent and thickness of the layer.

The electrochemical deposition of the Cu—Sn alloy for forming the layer 22 may be performed using, for example, the process as taught by Meudre, et al., “Elaboration of copper-tin alloys coatings: effect of glycine,” J. Mater. Environ. Sci. 6 (7) (2015), incorporated herein by reference.

Next, a tin or tin-based film 24 is formed, for example, using a plating process, over the layer 22 as shown in FIG. 2C. The film 24 may, for example, have a thickness on the order of 1 μm (more generally, a thickness in a range from 0.5 μm to 1.0 μm). In this regard, it is noted that a ratio of about 1:1 for the thicknesses of the layer 22 and the film 24 is preferred in order to ensure stability of the Cu—Sn alloy.

It is important to note some significant advantages of the process of FIGS. 2A-2C over the prior art process of FIGS. 1A-1C.

First, there is no need to perform any form of a thermal treatment. In this context, there is specifically no need to perform a thermal treatment after plating the tin or tin-based film 24, and this is advantageous because such “post bake” processing can lead to oxidation of the tin and give rise to solderability problems. Notably, the process of FIGS. 2A-2C supports the formation of a barrier between the copper base 20 and the tin film 24 that is achieved naturally at room temperature. In this context, it is noted that the diffusion mechanism changes from grain boundary to bulk at a temperature greater than two-thirds of the melting point of the lower of the two elements in the alloy. In this case, that would be at about 155° C. for tin, which has a melting point of 232° C. This difference between grain boundary diffusion and bulk diffusion as a function of temperature is critical to the formation of the desired uniform intermetallic Cu—Sn alloy layer 22.

Second, the electroplating of the copper-tin compound to directly form the layer 22 on the surface(s) of the copper base 10 advantageously produces a uniform layer, where prior art copper diffusion processing as shown in FIG. 1B can produce non-uniformities in the barrier film leading to an increased risk of whisker growth. In this regard, it is recognized that the Cu₆Sn₅ alloy will not uniformly grow at room temperature; it is through the use of the electroplating process that the uniform layer 22 is possible.

As used herein, the terms “substantially,” “approximately,” or “on the order of” are used to designate a tolerance of plus or minus 10%, more preferably 5%, of the value in question.

The foregoing description has provided by way of exemplary and non-limiting examples of a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims. 

1. A method, comprising: electrochemically depositing a uniform copper-tin compound layer on a surface of a copper-based base structure; and forming a tin-based film on the copper-tin compound layer.
 2. The method of claim 1, wherein the copper-tin compound layer comprises Cu₆Sn₅.
 3. The method of claim 1, wherein the copper-tin compound layer has a thickness in a range of 0.5 to 1.0 μm, and wherein the tin-based film has a thickness in a range of 0.5 to 1.0 μm.
 4. The method of claim 1, wherein a ratio of a thickness of the copper-tin compound layer and a thickness of the tin-based film is substantially 1:1.
 5. The method of claim 1, wherein the uniform copper-tin compound layer has a substantially constant thickness across said surface of the copper-based base structure.
 6. The method of claim 1, wherein the uniform copper-tin compound layer has a substantially constant stoichiometry over a lateral extent and thickness of the uniform copper-tin compound layer.
 7. The method of claim 1, wherein the uniform copper-tin compound layer has a homogenous granular structure with a smooth surface. 8-16. (canceled)
 17. The method of claim 1, wherein electrochemically depositing comprises electroplating.
 18. The method of claim 1, wherein the tin-based film is made solely of tin.
 19. The method of claim 1, wherein no post-bake process is performed.
 20. The method of claim 1, wherein a stoichiometry of the uniform copper-tin compound layer is set at room temperature.
 21. The method of claim 20, wherein the stoichiometry forms Cu₆Sn₅. 